1. Field of the Invention
This invention relates to a method for making an integral, laminate-like fibrous sheet material made preferably from predominately papermaking fibers and especially desirable for use as soft, absorbent, sanitary disposable towels and wipers.
2. Description of the Prior Art
A strong trend exists in the paper industry to develop disposable products from papermaking fibers and other fibers to serve as substitutes for conventional cloth products which are used as wipers and towels in both the home and industrial shops. To successfully gain consumer acceptance of these paper products, they must closely simulate cloth in both consumer perception and in performance. Thus, certain physical properties must be present in a successful product. These properties generally include softness, strength, stretchability, absorbency, ability to wipe dry, bulk and abrasion resistance. Depending upon the particular intended use of the product, some properties are more desirable than others.
Softness is one property which is highly desirable for almost all of the paper products regardless of their intended use. This is true not only because consumers find it more pleasant to handle soft feeling products, but also because softness enables the shape of the product to be readily conformable to the shape dictated by job requirements. Strength and the ability to stretch are two other properties which are desirable, particularly in those products which are to be used for heavy work duty. Also, it is desirable for the products to have good abrasion resistance if they are to be used for cleaning or scouring. If the primary functon of the particular product is to wipe function spilled liquids, absorbency and the ability to wipe dry are two other properties which are highly desirable. Bulk is important not only because it enables the paper product to feel like cloth, but also because it is favorably interrelated to other desirable properties, such as softness and absorbency.
Some of these properties are somewhat adversely interrelated to each other. That is to say, an increase in one property is usually accompanied by a decrease in another property. For example, an increase in web density or fiber concentration (the closeness of the fibers to each other) increases the ability of the web to wipe dry or pick up moisture, due to the greater capillary action of the small spaces between the fibers. However, an increase in closeness of the fibers decreases the spaces between the fibers available fo holding the moisture, and thus reduces the absorbency, in terms of quantity, of the web.
Perhaps an even more demonstrative example of the adverse interrelation between properties is represented by the relationship between strength and softness. It has generally been believed that conventional methods employed to produce soft paper necessarily result in strength reduction. This is because conventional paper products are formed from aqueous slurries, wherein the principle source of strength comes from interfiber bonds formed by the hydrate bonding process associated with papermaking. Paper which has a heavy concentration of these papermaking bonds is usually stiff. To soften the paper, it is necessary to reduce these stiff bonds, an action which also results in a loss of strength.
The method most commonly employed to reduce the stiff papermaking bonds is to crepe the paper from a drying surface with a doctor blade, disrupting and breaking many of the interfiber bonds in the paper web. Other methods which have been used to reduce these bonds contrast with creping by preventing formation of the bonds, rather than breaking them after they are formed. Examples of these other methods are chemical treatment of the papermaking fibers to reduce their interfiber bonding capacity before they are deposited on the web-forming surface, use of unrefined fibers in the slurry, inclusion into the slurry synthetic fibers not exhibiting the ability to form papermaking bonds, and use of little or no pressing of the web to remove the water from the paper web after it is deposited on the web forming surface. This latter method reduces formation of bonds by reducing close contact of the fibers with each other during the forming process. All of these methods can be employed successfully to increase the softness of paper webs, but only with an accompanying loss of strength in the web.
Attempts to restore the strength lost by reducing the papermaking bonds have included the addition to the web of bonding materials which are capable of adding strength to a greater degree than adding stiffness to the web. One method which has been used to apply bonding materials to the web is to add the bonding material to the aqueous slurry of fibers and deposit it on the web-forming surface along with the fibers. With this method, the bonding material can be distributed evenly throughout the web, avoiding the harshness which would accompany concentrations of bonding material. However, this method has the disadvantage of reducing the absorbency of the web by filling the pores between the fibers with bonding material. It also bonds the web uniformly throughout, the disadvantage of which will be explained subsequently.
Another method which has been used to apply bonding material to the web is to apply the bonding material in a spaced-apart pattern to the web. By this method, the majority of the web surface does not contain absorbency-reducing bonding material. This method is commonly employed in the field of nonwovens where little or no strength is imparted to the web by papermaking bonds, and almost all of the strength is obtained from the bonding materials. (Some of the strength may be obtained from intertwining of fibers, where the fibers are long enough to do so.) However, the fibers in such nonwoven webs are sufficiently long to enable small amounts of bonding material to impart substantial strength to the web, because adjacent areas of the bonding material in the spaced apart pattern can be quite far apart and yet be able to bond each fiber into the network.
In contrast to nonwoven webs, webs made entirely or principally from papermaking fibers require bonding areas to be quite close together because papermaking fibers are very short, generally less than one-quarter of an inch long. Thus, it has been thought that to apply sufficient bonding material in a pattern to a paper web to the degree necessary to bond each fiber into the network would result in a harsh sheet, having poor softness characteristics, particularly in the areas where the bonding material is located.
A method has been discovered which reduces the harshness in the web area where the bonding material is concentrated. That method is disclosed in U.S. Pat. Application Ser. No. 156,327 and, in its preferred form, consists of first forming a fibrous web under conditions which result in very low interfiber bonding strength by one of the previously described methods. Strength is then imparted to the web by applying bonding material to one surface of the web in a fine spacedapart pattern. The harshness in the bonded areas is reduced by tightly adhering bonded portions of the web to a creping surface and removing with a doctor blade, thus finely creping the bonded portions to soften them. This form of controlled creping also results in a number of other property improvements. For example, selective creping of the bonded areas in the surface of the web creates contraction of the surface of the web in all directions, resulting in an increase in stretch in both the machine direction and the cross-machine direction of the web. Also, the portions of the web where the bonding material is not located are generally disrupted by the creping action, resulting in an increase in bulk of the web, an increase in the softness of the web, and an increase in absorbency. At certain locations within the web, close to the bonding material, the web develops internal split portions which further enhance the absorbency, softness, and bulk of the web. It is this effect on the portions where the bonding material is not located which does not exist, at least to the same extent, in the web formed by addition of bonding material to the aqueous slurry of fibers.
This method produces a paper web with outstanding softness and strength, two properties which were previously believed to be almost mutually exclusive. It also produces a web with excellent absorbency properties due to the bonding material being confined to only a minor portion of the web surface. Furthermore, the compaction of the surface fibers due to the shrinkage of the bonded portions on the web creates one surface of the web which has improved wipe-dry characteristics. It is also believed that pressing the web to a creping surface while the web has moist portions in the surface region due to the uncured or undried bonding material causes the fibers in those moist areas to compact.
This method is particularly useful in production of webs in a lower basis weight range for such use as bathroom tissues. However, it has shortcomings in making webs for heavier duty use such as for towels where greater strength, bulk and absorbency is desired. Examples of such shortcomings are poor abrasion resistance and inability to hold to the web the fibers on the nonbonded side of the web, as well as less strength in the overall web than may be desired. These properties could be improved by causing the bonding material to penetrate completely through the web to create a network of bonding material which passes entirely through the web, but the web would be subjected to a lesser extent to the improvements in the properties afforded by practice of the Ser. No. 156,327 invention. For example, bonding the web with the bonding material extending completely through the web would greatly reduce the disruption of the fibers within the web upon creping, and therefore, result in a reduction of bulk, softness, and absorbency. Also, complete penetration of the bonding material through the web is difficult to accomplish on heavier basis weight webs and attempts to do so result in concentrations of excess bonding material at the web surface where much of it is ineffective for strengthening interfiber bonds. Furthermore, if complete penetration of the bonding material does result, the bonding material in the interior of the web will not be as efficiently used to increase abrasion resistance of the web as when it is placed only in the surface of the web. Placement of the bonding material in the interior of the web is not only an ineffecient use of the expensive bonding material, but results in harsher feel to the web due to the inability of the creping action to soften the bonded portions as effectively.
Also, one desirable feature of the Ser. No. 156,327 invention which would be reduced by bonding completely through the web is the ability to create a web surface of compacted fibers having good wipe-dry characteristics while at the same time creating a bulky web capable of absorbing a large amount of moisture. These properties are only of minor importance when producing a product for such uses as bathroom tissues, but where the product is to be used for wipers or towels, it is very important. In reference to this point, it should be noted that the product of the Ser. No. 156,327 invention has an additional shortcoming in regard to use as wipers or towels in that it exhibits good abrasion-resistance only on one side of the sheet, the side on which the bonding material is applied. This shortcoming detracts from the Ser. No. 156,327 invention as a method of producing a wiper or towel product, especially one which requires considerable work abuse.
Probably the most commonly employed method to produce a wiper-like paper product having the desirable bulk, absorbency, and abrasion-resistance, is to laminate two or more embossed conventional paper webs together with an adhesive. One advantage of this method is that the tightly compacted fibers of the conventional paper webs offer good wipe-dry properties on both sides of the sheet while at the same time, the void spaces between the webs created by the embossments spacing the webs from each other increase the ability of the web to hold moisture. Examples of this method are disclosed in U.S. Pat. No. 3,414,459 and 3,556,907. The disadvantages of this method are apparent when considering the complex process involved in separately embossing two or more webs and then bringing them together with synchronism to prevent nesting of the embossed protuberances of one web into the embossed protuberances of the other web. Also, any given length of the multi-ply product requires initial production on a papermaking machine of a web two or more times as long. Furthermore, prior art multi-ply paper products have generally been made from conventional paper webs which depend almost entirely upon stiff papermaking bonds for their strength.
Notwithstanding these shortcomings, multi-ply paper products are quite desirable in that they can be made very bulky compared to their weight, due to the void spaces between the plies created by the embossed protuberances holding the plies apart from each other. Because of this construction, multi-ply products are easily compressed between the fingers of the consumer, thereby aiding in giving some feeling of softness.
A consideration of the advangtages of multi-ply products and the disadvantageous harshness of prior art multi-ply products would suggest making a multi-ply product by laminating two or more webs produced by the method disclosed in U.S. Pat. application Ser. No. 156,327. However, the prior art methods of laminating multiple plies of webs together involve the undesirable requirements of a complex laminating process and the necessity of producing a greater length of initial web for laminating, both reducing the advantage of such a suggested product.
From the foregoing discussion, it can be seen that it would be very desirable and is, therefore, an object of the invention to produce a paper product which has the bulk, absorbency, and compressability of a conventional multi-ply paper product, but without the usual accompanying lack of softness due to their extensive dependence upon papermaking bonds for strength. It would also be very desirable and is, therefore, an object of the invention to produce a paper product with the desirable properties of a multi-ply product without the disadvantages of the complex process necessarily employed in the prior art. These desirable features are provided in the product of the invention produced by the method of the invention.